Fuel cells come of age

The need for clean energy has led to the development of many new technologies. John Franceschina of FuelCell Energy, explores how one such development has matured and its potential benefits.

The vast majority of electrical power is produced by centralised power plants – primarily natural gas-, oil-, and coal-fired. Unfortunately, these power stations give off large amounts of harmful pollutants and greenhouse gases. Combined cycle plants use residual heat to improve overall power generation efficiency. However, the distance to consumers makes it difficult to otherwise utilise combined heat and power (CHP) effectively. Transmission networks carry energy to the consumers, sometimes hundreds of miles away and line losses further degrade overall efficiency. Fortunately, advances in distributed generation technologies – in particular, fuel cells – offer solutions for primary (baseload) power that can augment the grid in ways that improve efficiency, reliability, and environmental impact.

Distributed generation vs the power grid

Today, most of the electricity produced in the US is provided by regional utilities and supplied to customers via the grid. As a matter of fact, of the 3800mMWh of electricity produced in the US in 2003, only 4.1 per cent (156mMWh) was non-utility generation. Commercial utilities produced 3.6 per cent (135mMWh) to meet local power needs, and the remaining 0.5 per cent (21mMWh) was attributable to other sources. Wind, solar, hydro and other renewable energy sources augment the grid and are making a respectable contribution. Much of this energy, however, is vulnerable to the whims of nature and thus difficult to manage for the utilities. This is the case for wind and solar in particular.

Slowly, however, the landscape is changing, as distributed power generation becomes more practical. Commercial businesses and institutions such as hotels, universities, and government facilities, to name but a few, are moving toward energy independence and reduced reliance on the grid. Doing so provides a degree of flexibility not otherwise possible and reduces the grid congestion and power transmission issues associated with centralised generation. There is also another advantage of distributed generation: proximity to the consumer. This provides an opportunity to use CHP effectively, thus reducing overall energy costs and boosting the efficiency of the process considerably. Fuel cells also offer the unique advantage of compactness, competitive operating costs, and “ultra-clean” emissions. They also operate 24/7 and so are easily managed in concert with grid power.

Why fuel cells?

With power availability rated in excess of 95 per cent and an electric power generation efficiency far exceeding other processes, fuel cell technology has advanced to the point where it is now a viable alternative to combustion-based plants for a growing number of baseload power applications. Today, fuel cells are reaching their potential as the cleanest and most reliable source of distributed power generation.

Historically, fuel cells have been limited in practicality because of the need for a supply of hydrogen to operate. Certain systems, however, known as direct fuel cells, or DFCs, have been developed and are unaffected by such a limitation, as they operate on natural gas, biogases (from food processing and wastewater treatment) and propane. They have even been shown to generate clean power from diesel fuel and coal gas, fuels traditionally associated with high pollution. How is this possible? The systems internally reform hydrogen from the source fuel. Whatever the fuel source, DFCs emit dramatically reduced CO₂ greenhouse gas compared with combustion alternatives, and only negligible amounts of pollutants, such as NO_x and SO_x.

How direct fuel cells work

In essence, fuel cells are electrochemical devices that combine fuel with oxygen from the ambient air to produce electricity and heat, as well as water. The non-combustion process is a direct form of fuel-to-energy conversion and is much more efficient than conventional heat engine approaches. CO₂ emissions are reduced, due to the high efficiency of the fuel cell and the absence of combustion negates the production of NO_x and SO_x pollutants.

Fuel cells incorporate an anode and a cathode with an electrolyte inbetween, similar to a battery. The material used for the electrolyte and the design of the supporting structure determine the type and performance of the fuel cell. The DFC uses a carbonate-based electrolyte, which operates at high enough temperature to allow generation of hydrogen within the cell. Fuel and air reactions for the DFC occur at the anode and cathode, which are porous nickel (Ni) catalysts. The cathode side receives O₂ from the surrounding air.

Fuel (typically methane from natural gas or biogas) is supplied to the anode, which reforms the fuel into H₂. The gas is then consumed electrochemically. The O₂ supplied to the cathode, along with CO₂ recycled from the anode side, reacts with the carbonate salt electrolyte to produce carbonate ions that pass through the electrolyte to the anode, where they combine with the H2 to produce water, CO₂ and electrons. The electrons flow through an external circuit to the cathode, thus producing the desired power.

Figure 1: Block diagram of a direct fuel cell power plant. HRU is an acronym for hydrogen reforming unit.

Fuel cells and CHP

DFC power plants have an exhaust temperature ranging from 650°F to 750°F. This heat energy can be captured to provide heat for buildings, swimming pools, and other facility needs. In fact, the already high efficiency of fuel cells can be increased from around 47 per cent to more than 80 per cent by the use of thermal energy. Alternatively, the heat can be used with a turbine generator to convert the heat to electrical energy. Simple systems are commercially available today which can add 2-3 percentage points electrical efficiency by converting some of the thermal energy to electricity. More integrated fuel cell/turbine hybrid systems are under development which will operate with about 60 per cent electrical efficiency.

Fuel cell plants are typically located within or near to the facility where the electricity is to be used. This is a distinct advantage over conventional central plants that are usually located too far for effective utilisation of waste heat.

There are also certain other CHP considerations regarding the trade-off between heat and electricity that highlight the benefits of fuel cells over turbine and other combustion generators. Electricity generated during a cogeneration process has a significantly greater value than that of the associated waste heat, in fact, up to 10 times as much. Thus, the generation of electricity is paramount in the economic efficiency equation, since the more electricity that can be produced by the power plant, the less must be purchased from the grid.

With traditional sources of distributed power generation – eg, reciprocating engines, microturbines, etc – CHP can mask the underlying electrical power generation efficiency of the power source. Whatever CHP adds to the overall efficiency, there is no getting around the actual electrical power-generating efficiency of the plant. Thus, in the case of a microturbine, eg, operating at typically less than 30 per cent electric power generation efficiency and a reciprocating engine at 35 per cent electric power generation efficiency, considerably less of the overall output of the system is in the form of electricity. In contrast, the DFC operates at 47 per cent electrical power generation efficiency.

The bottom line is this: DFC fuel cells offer the distinct advantage of a higher ratio of electricity to heat – electricity that would be relatively expensive, if it had to be purchased from a grid – while capturing much of the heat generated by the CHP process for productive use. The higher ratio of electricity to heat also means that DFC operation is more effective at reducing carbon emissions compared to CHP systems with lower electrical efficiency.

Baseload power applications for stationary fuel cells

Fuel cell power plants are uniquely well-suited to a variety of distributed generation applications. In particular, hotels, food and beverage processing plants and wastewater treatment plants are discussed below. In addition, however, DFC power plants are in operation at wastewater treatment plants, manufacturing facilities, universities, hospitals, correctional institutions, government facilities, and even as pure grid support applications.

Take, for example, food and beverage processing. Digester gases are produced by the digestion of organic matter (which is done to reduce the amount of solid waste produced at a facility). Fuel cells can use the methane-containing digester gas to produce electricity and heat (which is used in the digesters). This avoids the need to flare unused gas and it provides the power much more cleanly than if the gas were used in a combustion-based generator.

The Sierra Nevada Brewing Company in Chico, California, has installed a 1MW DFC power plant to address its clean energy needs. The system is fuelled by digester gases given off in the beer production process and augmented with natural gas. The plant provides virtually all of Sierra Nevada’s baseload power requirement, of which about 40 per cent is produced using digester gas. The DFC power plant converts the digester gas into the most electricity possible by distributed generation technology, thereby maximising this limited resource. The result is high quality, utility-grade electric power, usable heat, and ultra-clean emissions. Waste heat is used to produce steam for the brewing process. The overall energy efficiency for a DFC plant is twice that of power supplied from the electrical grid.

The Dublin San Ramon wastewater treatment plant in Pleasanton, California, has a 600kW fuel cell power plant onsite. Sludge from the conventional sludge treatment process is anaerobically digested to reduce the volume of solid organic manner, producing methane gas in the process. The fuel cell receives the treated digester gas and converts it into electricity. Electrical power produced in the process is used to operate the plant, and heat produced by the fuel cell is recovered and used to heat the sludge, thus optimising the anaerobic digestion process.

Figure 2: 600kW Direct FuelCell installation at the Dublin San Ramon Services District uses anaerobic digester gas to power the wastewater treatment process.

Fuel cells are also helping many states in the US meet their recently-adopted Renewable Portfolio Standards (RPS), which have set electricity providers the task of obtaining a minimum percentage of their power from renewable energy resources by a certain date. Currently there are 28 states plus the District of Columbia that have RPS policies in place. Together, these states account for more than half of the electricity sales in the United States. Connecticut has turned to the Connecticut Clean Energy Fund (CCEF) to administrate their RPS programme. The CCEF, created and funded by the state legislature to encourage wider installation of clean energy technologies for the benefit of Connecticut ratepayers, established “Project 100” to encourage the installation of 100MW of electricity generated by renewable means by 2008.

After careful review of various proposals utilising an array of renewable energy technologies, the CCEF and the Department of Public Utility Control (DPUC) selected Direct FuelCell power plants from FuelCell Energy for installations totalling 16.2MW of Class I renewable energy. The project consists of three main fuel cell installations – a 2.4MW installation at Stamford Hospital, a 4.8MW installation at Waterbury Hospital, and a 9MW hybrid fuel cell/turboexpander installation at a gas pipeline transmission centre in the city of Milford. The success of the Project 100 scheme has led the CCEF to expand the RPS programme to cover 150MW of renewable power generation by 2020, through the recently announced “Project 150”.

Over in California, the state’s self-generation incentive programme (SGIP) has been amended so that fuel cell operators can benefit from subsidies for plants of up to 3MW in size, further demonstrating the desire of US policy-makers to support this promising new technology. The SGIP previously reimbursed fuel cell power plant owners US$4500/kW for biogas-run units and US$2500/kW for those operating on natural gas for installations of up to 1MW. Under the revised scheme, the first 1MW of a project is entitled to 100 per cent of the incentive, while the second and third megawatts are eligible for 50 and 25 per cent of the subsidy, respectively.

Another development, which underlines the appeal of this new technology among independent power producers (IPPs), is the order for 25.6MW of FuelCell Energy’s power plants and fuel cell modules from POSCO Power, FCE’s South Korea distribution partner. POSCO will be siting these units in various IPP and utility applications within South Korea. This brings POSCO’s total purchases of FCE products to date to 38.2MW.

The new plants and modules are expected to be delivered by mid-2009 and will make a positive contribution in the ongoing effort to reduce the country’s dependence on imported fuel. At the same time, this new development will help to decrease the emissions of greenhouse gases and other pollutants associated with power generation. It has effectively doubled FCE’s project backlog and represents almost US$70m in sales.

Fuel cells in natural gas transmission

In addition to providing efficient, distributed baseload power, fuel cells can be used to harness energy that would be otherwise wasted in the transport of natural gas via pipelines. The gas is transported over long distances at high pressure, which must be reduced before it reaches its final destination. The expansion in volume of the gas can be used by a turbine (also known as a turboexpander) to drive an electric generator. The process requires that the fuel be pre-heated to prevent excessive cooling during expansion and by using waste provided by the DFC, emissions that would be created by combustion technologies are eliminated. The combination of the DFC and pressure energy recovery generation (ERG) leads to greater levels of efficiency, lower pollutant emissions and higher capacity factor.
This approach has been developed by Enbridge Inc, a leading North American energy transportation and distribution company, and FuelCell Energy, resulting in the DFC-ERG (Direct FuelCell – Energy Recovery Generation) system, which produces up to 10MW of utility-grade power generation, with an electricity efficiency of up to 65 per cent, negligible NO_x and SO_x emissions and dramatically reduced CO₂ output as compared to traditional fossil-fuelled power generation. The system, in some markets, can offer a lucrative revenue stream from the export of electrical power to the utility grid and the sale of renewable energy credits.

Figure 3: The DCF-ERG system: The generation of power generation
via natural gas decompression.

Conclusions

With few exceptions, the worldwide community of nations recognises the impact that pollution is having on the environment. As a result, will coal and other fossil fuel power plants eventually disappear? Not likely. Certainly, however, there will be a continuing effort to augment centralised power generation with renewable sources of energy and on-site distributed generation.
The benefits of on-site power include grid congestion relief, higher efficiency through reduced line losses and CHP, and emissions reductions through the use of ultra-clean technologies. While traditionally distributed generation has been viewed primarily as backup power, fuel cells offer the opportunity to produce baseload power 24/7 with negligible emissions and dramatic reductions in greenhouse gases. Fuel cell power plants are beginning to take centre-stage for distributed generation of baseload power in a variety of commercial, industrial and utility markets.

John Franceschina is vice president of business development for FuelCell Energy. He has more than 18 years of experience in the energy industry.

For more information about FuelCell Energy’s range of products, visit their website here.